Counterintuitively,
Avoid Weapon Special Abilities that Inflict Damage
If your PC has been taking feats like Favored in House (ECS 53), and Least Dragonmark (mark of finding (ECS 64)) (ECS 53), and Skill Focus (Bluff) (PH 100) so he can better make Seduce to Learn Secret checks while the other PCs have been specializing in monster murdering, and they realize you aren't contributing much in a fight, explain that you'll need their help. As a swashbuckler (who doesn't cast spells) you were going to need it anyway. That's not an insult but a fact of mid- and high-level play.
You'll need their help because the first weapon special ability you add to your rapier is spell storing (DMG 225) (+1 bonus). No other weapon special ability scales for damage as well as it does because it scales with your party, one of whom should be happy to refill your +1 spell storing rapier free of charge at the end of a slow adventuring day. If you must fill your own tank, take ranks in the Use Magic Device skill and load your +1 spell storing rapier using a wand of combust [evoc] (SpC 50) (2nd-level spell at caster level 10) (300 gp/charge) or a wand of vampiric touch [necro] (PH 298) (3rd-level spell at caster level 20) (900 gp/charge) or a wand of another spell that the Comments will inevitably suggest.
It sounds like you're in a game wherein sources of magic items are restricted. If you aren't, also consider the weapon special ability bloodstone (MIC 29) (+1 bonus), which is like the weapon special ability spell storing that only holds the spell vampiric touch but the touch is modified by the feat Empower Spell (PH 93) for free. Seriously. Then add the weapon special ability power storing (MIC 40) (+1 bonus) if there's a psion available; my unfamiliarity with psionics means I don't know what's best, but there must be something awesome and Eberron is psionics-friendly. Then virtually guarantee your attack hits by adding the weapon special ability impaling (MIC 37) (+1 bonus).
If you are confined to the Dungeon Master's Guide for your weapon, the only other serious choices for damage are the weapon special abilities bane (DMG 224) (+1 bonus), picking whatever the DM's throwing at you a lot, and holy (DMG 225) (+2 bonus) or, y'know, unholy (DMG 226) (+2 bonus) if you're an evil swashbuckler: a lot of your foes will be evil (or good) anyway; punish them for it. If chasing just damage buy the weapon special abilities magebane (CAr 143-4) (+1 bonus) and vampiric (MIC 45) (+2 bonus) if allowed.
The flat +5 damage from the weapon special ability collision (MIC 31) (+2 bonus) is the best you're going to get, but also consider, if it's permitted, the weapon special ability fierce (AE 96) (+2 bonus) that lets the weapon's wielder to take a penalty to his Armor Class up to his Dexterity bonus to add the the penalty's absolute value as a bonus to the weapon's damage.
Although others may disagree, most of the other damaging effects from the DMG and elsewhere inflict energy damage that a high level creature's energy resistance will easily absorb, employ effects high level foes are often immune to (e.g. ability damage and ability drain, critical hits, negative energy effects), or have saving throw DCs that are too low to be viable.
Effects That Don't Inflict Damage, But Take a Look
The weapon special ability dispelling (MIC 33) (+1 bonus) et. al. will make you popular with your group even if you aren't inflicting a lot of damage. The same goes for the weapon special ability revealing (MIC 42) (+1 bonus), which eliminates miss chances for your party; in high-level play miss chances are commonplace, and everyone will thank you even if you hit only once.
The elemental power weapon special abilities from DMG2 254-5 et. al. range from a +1 bonus to a +4 bonus and permit summoning of an elemental of increasing size based on the bonus purchased. Being an elemental-summoning swashbuckler may sound strange, but the value of minions with cool powers can't be overstated--ask any necromancer.
I don't like abilities that allow saving throws nor do I like abilities relying on critical hits, but the weapon special ability prismatic burst (MIC 40) (30,000 gp) is an exception to both. The flat cost means it's possible to add it to a weapon that's already full of other bonuses. The weapon special ability sending (LE 152) (+4 bonus) is also valuable, despite only activating on a natural 20 and a successful critical roll; a no-saving-throw teleport effect versus a high-level foe is more likely to remove him from battle than almost any other effect, although by the time this is avaialable most foes should have a way of returning to the battlefield shortly.
I've not heard of a DM actually allowing the weapon special ability smoking (LD 180) (+1 bonus), but I suggest it because it exists. It's a defensive ability, but a spectactular one, and defensive abilities keep you alive long enough to use offensive abilities. The weapon special ability spellblade (PG 120) (6,000 gp) is just as cool; pick a targeted spell the DM loves, ignore it, and even huck it back at the caster.
The weapon special ability Keeper's fang (ECS 266) (+4 bonus) renders creatures you kill with the weapon unable to return from the dead via the spells "resurrection, raise dead, reincarnate, or similar effects"; the weapon becomes evil and makes you look like a jerk to some folks, but this is such a powerful ability in any typical Dungeons and Dragons campaign that its utility can't be ignored.
Don't Neglect Accessories!
A broken weapon made from aurorum (BE 38) (+4,000 gp) can be made whole as full-round action, but a weapon made from riverine (Sto 128) (+2,000 gp/lb.) is just immune to damage.
A weapon made from morghuth-iron (AE 14) (+4,000 gp) is constantly naturally poisonous, but the poison's saving throw DC is low, the weapon's wielder takes a -1 penalty to attack rolls and damage rolls with the weapon, and many high-level creatures are straight-up immune to poison. This is, however, far cheaper than many effects, and there's no use limit.
A weapon with the template pitspawned (DMG2 278) (+1,000 gp; 0 lbs.) grants the wielder a +2 bonus to critical rolls.
A wand chamber (Du 34) (100 gp; 0 lbs.) is a space in the weapon for a wand which can be used without drawing the wand; perfect if you've ranks in the Use Magic Device skill. A hilt hollow (Du 33) (300 gp; 0 lbs.) is a space in the weapon for a potion which must be extracted but it's easier than normal. Get both. An oil chamber (Du 33-4) (1,000 gp; 0 lbs.) is expensive, but can hold an oil of greater magic weapon [trans] (PH 251-2) (3rd-level spell at caster level 20) (DMG 230) (3,000 gp; 0.1 lbs.) or ghostoil (AE 34) (50 gp; 1 lb.). A triple weapon capsule retainer (CAd 121-2) (450 gp; 0 lbs.) holds 3 alchemical weapon capsules (CAd 122), which do everything from more damage to silvering your weapon on the fly.
You'll want a weapon with at least a +3 enhancement bonus (not just a +1 weapon with at least 1 additional +2 bonus or 2 additional +1 bonus weapon special abilities!) if planning to attach the most powerful augment crystals (MIC 221). Start with the crystal of return (least) (MIC 65) (300 gp; 0 lbs.) now if you've not the feat Quick Draw (PH 98) or another way to draw a weapon as a free action. Purchase more augment crystals as needed for the campaign.
Suggestions
All bets are off if the campaign allows more sources than these.
- A +1 bane (humans) holy spell-storing adamantine rapier (53,320 gp; 3 lbs.) if you can only use the Dungeon Master's Guide.
- A +1 bloodstone collision dispelling impaling prismatic burst spell-storing vampiric adamantine rapier (195,320 gp; 3 lbs.) if you can use the Dungeon Master's Guide and the Magic Items Compendium. Perspective: This weapon costs nearly two tons of gold.
I'll try to get the simpler issues done first - and the simplest one is Great Weapon Fighting. You should not take this. If you're using a spear, it is strictly inferior to the Duelling Fighting Style. See my answer here for the analysis. It's especially true for you because you want to tank, and for a quick and easy boost to your tankiness it's hard to go past a shield.
The next issue is how many levels of Paladin to take. This one gets a lot more complicated, but I'll start with the basics. When multiclassing in 5e, you want to take either 1 level or a number of levels that is a multiple of 4. This way, you don't lose any of the vitally important ability score increases.
The next thing to consider is what you actually want to get out of your Paladin levels, and what each number has to offer you. You also need to consider what Bard features you're missing out on by taking Paladin levels, but you've already got almost all the Bard features - from here on out your existing abilities mostly just get more powerful, rather than you gaining new things.
Paladin level 1: Gives basically nothing. You get the multiclassing proficiencies, Divine Sense, and Lay on Hands. Not a useful number.
Paladin level 4: Fighting Style, Divine Smite, Divine Health, and Channel Divinity. (And 1st-level Paladin spells, for what that's worth.) This is pretty good - a lot of the Paladin's core features for a minimal investment. If you're taking 4 levels, I'd recommend the Oath of Devotion, since (I think) it has the best Channel Divinity options.
Paladin level 8: All of the above, plus Extra Attack, Aura of Protection, and either Aura of Devotion or Aura of Warding. (Technically you could take Relentless Avenger, but it's not that great, especially for what you're trying to do.) Also 2nd-level Paladin spells, which is cool because one of them can be Find Steed, which is basically a class feature masquerading as a spell. This is a good place to finish taking Paladin levels because the next really good feature is at level 15. If you're going this far, the Oath of the Ancients starts looking as good as the Oath of Devotion, because Aura of Warding is so powerful.
I'd definitely recommend either 4 or 8 levels of Paladin if you're going to do this - 1 just seems like a waste of time. As for what order to take levels in, I'd actually suggest waiting till you've got your next ability score increase to do this, because doing it without War Caster might be a little awkward. Obviously, if you want to get better in melee sooner rather than later, you should start taking Paladin levels sooner rather than later, and you can work around the problems with casting your Bard spells with your hands full, it's just tricky.
Best Answer
So I was going to recommend horizon tripper, as DanB has in his answer, since that is about as good as a core-only martial character can do, and that remains a relatively good choice—but since he’s already offered it, I’ve been thinking about alternatives. You have stiff competition in the form of the cleric and druid, and a horizon tripper is probably not going to keep up. Your source list also does you almost no favors—beyond core, you have a magic-focused book (Tome of Magic), a mostly-fluff book (Planar Handbook), one of the worst books WotC published for 3.5 (Book of Exalted Deeds), and a third-party supplement that is also pretty poor (Guidebook to the Planetouched). You would be far better off with more typical supplements like Complete Champion, Expanded Psionics Handbook, and Unearthed Arcana, for examples.
But those books do, just barely, allow someone who is big and tough to stand in front of spellcasters, and just ruin the day of anyone that they have a problem with. The name of the game is killing things’ saving throws with auras, so they are easy pickings for your spellcaster allies, and then holding them there. If it works, you could very easily be a spellcaster’s best friend.
It just so happens that Tome of Magic is one of the best sources in the game for these kinds of auras—two binder vestiges, Focalor and Chupoclops, allow you to reduce enemy saving throws by −2 each. And one of the other best sources is the core blackguard prestige class, though this may or may not stack with Chupoclops (both auras are called aura of despair, unclear if that means they are the same source for modifier stacking).
But binders and blackguards are Charisma-based. Dark blessing, in particular, rewards you heavily for Charisma. And your Charisma is one of your worst scores. You do have a solution to this, though: Guidebook to the Planetouched does have the extremely ill-advised Thin Blood feat. This feat reduces LA by 1—an incredibly bad idea, and ridiculously overpowered. It actually would do a lot to help a horizon tripper if you had more sources available, since you could get powerful build or Large size without LA by using it. But none of the decent options for those things is available to you (for reference, the best options for horizon tripper that are available to you are almost-certainly wildren or air mephling). So we are looking for a LA +1 race that gives a Charisma bonus.
You also have 13 Constitution. This is a death sentence. You are looking to stand up in front of enemies, cripple their defenses, and make it difficult for them to get away or around you. If you aren’t drawing a ton of hostile attention, something is very wrong. But even a regular character would have a hard time surviving on just 13 Constitution. A character like you will simply die, probably quickly, because it just isn’t enough. So a bonus to Constitution would also be very important.
Luckily for you, there is one race in your sources that does fit the bill: the water mephling from Planar Handbook. This race is otherwise pretty useless for you—humanoid, so no useful type; small, which hurts your damage and makes it harder to get reach from size increases; a breath weapon, which you should never use; a swim speed, when your air and earth cousins have the fantastically-superior flying or burrow speeds. But you absolutely need those ability scores. Actually, you need more than even this—15 of each in what are probably your most important scores is not ideal.
You’re high enough level to bump those to 16 (the third point should go to Strength), and get a cloak of charisma and periapt of vitality to boost them, too. These are good, and should be your top priorities for wealth (even before a belt of giant strength).
That means we have a couple of questions for the DM:
If the answer to the first is “they stack,” then trying to get both blackguard and 10th-level binding is probably worthwhile, for that huge −6 penalty to saving throws. If not, then we can and should ditch either blackguard or most of the binding (just getting Focalor only takes 3rd-level binding). Blackguard uses far fewer levels, but by not being a blackguard we can kind of go the other way on things and use champion of Gwynharwyf from Book of Exalted Deeds, which gets a delicious rage and divine grace combo. So I have three builds to suggest here, but first I want to point out an excellent feat for all three of them:
Planar Touchstone
I want to draw your attention to Planar Touchstone in Planar Handbook. This feat can do many, many things, depending on what site you attune to. A particularly good option is the Catalogues of Enlightenment, which can give you a cleric domain—lots of good options there. This is, quite frankly, one of the best feats in the game. It doesn’t directly enable any builds, but you can fit it into any of the builds below. And you’re playing in Planescape: what a natural choice for such a game.
So I strongly recommend Planar Touchstone. Here’s a thread collecting the best uses of it.
The Builds
These are the three builds that I imagine will work. They’re pretty similar to one another, variations on a theme, really.
Eternal Guard of the Black Tears
We need 10th-level binding, and we need a 3rd-level blackguard feature. You start at 13th level. This almost writes itself.
Any¹ evil water mephling with the following:
We go with straight binder here, rather than knight of the sacred seal, because 1. feats are at a premium here, and although we could technically get Weapon Focus, 2. we don’t have martial weapon proficiency until we start blackguard, and that means the Weapon Focus feat would have to be for longspear or something. Better to take Weapon Focus at 12th or 15th where we can pick a better weapon—probably guisarme—and take knight of the sacred seal later.
If you cannot qualify as having Improved Sunder with Aym, you have to take Improved Sunder as well: that will have to replace Combat Reflexes, which you may want to do for your 12th-level feat in that situation.
If you cannot stack two different auras of despair, then Chupoclops is not worth using—but this build could still be valid. Zagan, for example, is good at grappling—that could help you keep people inside the aura. Zceryll is from a web enhancement and is thus presumably not valid here, but if you can get her she is definitely overpowered and spamming summons can do a lot to control where people can go in combat. Haures can spam illusions, potentially keeping people where you want them, and Vanus’s fear aura is both more dangerous thanks to the save penalties, and can keep enemies away from allies. Dantalion offers even better mobility than horizon walker, Balam is solid defensively, and so on. Lots of options. Still, pretty big cost with all those binding levels.
Finally, one last note: blackguards get poison use. Poisons pretty much suck, and you don’t have the supplement support you really need to do well with them... but penalizing saving throws makes people way more vulnerable to poison, just as it does spells. Even if you just carry a few cheap poisons and start combat with a poisoned weapon—without re-applying mid-combat—could be worthwhile.
As for continuing play after 13th, your best bet is probably just continuing binder levels. If you go with Weapon Focus at 12th or 15th, then you can use knight of the sacred seal instead for five of the remaining levels. You get a 3rd vestige at 17th, and at 18th you actually get 8th-level vestiges. That’s better than anything blackguard, or anything else really, has to offer.
However, if you don’t think you will get to 17th level, you are probably best off just going with blackguard (none of its remaining features are thrilling but they aren’t terrible, and binder gives you little-to-nothing until you can bind another vestige).
Starry Knight-Harbinger
Here we need rage, 10th-level binding, and a couple levels in champion of Gwynharwyf. Once again, this almost writes itself.
Chaotic good water mephling with the following:
We are again not taking knight of the sacred seal—simply because this time, we can’t, since it requires nonchaotic and champion of Gwynharwyf requires chaotic good—but really, that doesn’t lose us all that much.
Ultimately, the difference between this build and the previous is that we trade a second aura of despair for rage. Rage is very nice, but the aura is far better. But if you can’t stack the two auras of despair anyway, this is not a bad choice.
As for the rest of the build, you can could choose to take more binding levels, as in the previous build—but with no hope of adding knight of the sacred seal. Personally, I think I’d just finish out with champion of Gwynharwyf. You have the best of its features already, but its remaining levels are decent enough. You just miss the capstone, but c’est la vie. Even though you can manage a 3rd vestige and 8th-level vestiges, champion really is quite good and damnit, you’ve paid quite a bit to get into it.
###Forlorn and Forsworn
This approach saves us a lot of levels: instead of 10th-level binding, we only need 3rd-level binding, plus a 3rd-level blackguard feature. What to do with the other seven levels? Well, we desperately need feats so a level or two of fighter might be good. Rage remains excellent, so a level of barbarian seems appropriate. Beyond that, the elemental warrior prestige class in Planar Handbook is... largely garbage, but the 4th-level feature can get us flying, which is important, and the class is dead easy to enter. That’s probably better than anything more levels of fighter or blackguard would get us.
Chaotic-or-neutral evil water mephling with the following:
Choose air, it’s by far the best option. Water has the best elemental strike, but air is probably second in that regard, and air absolutely wins in manifestation and most importantly, movement. And yes, that means you are a water mephling with an air affinity—write a nice backstory for it, but don’t give up on it. You need to fly.
Planar Touchstone is almost-certainly the best option, but you could take Endurance. It’s awful, but then you could finish out with horizon walker and just reach shifting planar terrain mastery, which is nice, ish. You could also take Planar Touchstone at 15th in such a situation.
If you cannot qualify as having Improved Sunder with Aym, you have to take Improved Sunder as well: that will have to replace Combat Reflexes, which you then have to move until 12th, eliminating your options there.
Anyway, compared to the first build, this build “only” gets −4 to enemy saving throws, but the flight speed and ability to actually get Improved Trip go a long way towards mitigating that—it will be easier to get people inside the aura and keep them there. And if you can’t stack the two auras of despair, this may be a better approach than the things you can do with other vestiges.
And again, as with the first build, note the poison option.
Your 14th level here should definitely be elemental warrior, to get that sweet flight speed. After that, you’re kind of wide open: six levels to fill as you like. Six levels of horizon walker gets shifting planar terrain mastery: nice, ish. Or you could get a second vestige with five more levels of binder (or knight of the sacred seal; Weapon Focus could be your 15th-level feat). The rest of blackguard isn’t stellar, but a couple levels gets you 3rd-level spells, some sneak attack, and 3rd-level spells. Dipping cleric for domains could be nice, though you have somewhat limited options. Another level of barbarian would be uncanny dodge: not bad. Ranger has a lot of skills with full BAB, though pretty much everything else about it is worthless. For that matter, you don’t need full BAB, so you could do rogue or assassin if you wanted.
Conclusion
There are a lot of problems with this game, and you are probably much better off just going with a full spellcaster. Being a spellcaster will make it nearly impossible to really take advantage of your ability scores (I guess you could burn a spellcasting level on eldritch knight—being an outsider gets you the proficiencies you need—but it’s not really worth it), which is a shame, but it still probably ends up being better. In fact, the choice to start so high, the selection of sources, and most of all, the extremely poor choice to enforce rolling ability scores in order, are a number of serious red flags to me—they suggest to me that this DM is not very familiar with how 3.5 works, doesn’t realize or understand how much these parameters all-but-force you to play a spellcaster, and that is a concern because keeping a campaign that high-level working takes a lot of work even for a very good DM.
But if we ignore all that, and you want to really use your ability scores, standing in things’ way and tanking their saving throws might do a lot for your party. Any of these builds should be reasonably resilient, since they add Charisma to all saving throws and can use armor and have decent hp. And tacking a −4 or −6 penalty to nearby enemies’ saving throws can make you everyone’s best friend.